In order to ensure the safety of the passengers and crewmembers of an aircraft, in case of pressurization failure, the occurrence of smoke or other trouble affecting the gas contained in the air, aviation regulations require on board airliners a safety oxygen supply circuit able to supply passengers and crewmembers with an oxygen enrichment function of the aircraft altitude.
It is already known, in particular from WO-2009/007794, a breathing assembly for aircraft comprising:                a breathing mask to be connected to a source of breathing gas,        a stowage box forming a receptacle for receiving the breathing mask in a stowage position, said stowage box having an opening for extracting the breathing mask from the stowage box, and        a valve assembly comprising a valve having an open position in which the valve allows the feed of breathing gas to the breathing mask and a closed position in which the valve prevents the feed of breathing gas to the breathing mask, the valve being automatically positioned in the open position when the breathing mask is extracted from the stowage box.        
After a pressurization failure or appearance of smoke, the crewmembers and in particular the pilot must extract the breathing mask from the stowage box and don it upon their face to be fed with oxygen. The breathing mask must be donned quickly, since oxygen lacks at high altitude (hypoxia) or toxic smoke may alter the abilities of the crewmembers to proceed with any emergency measures that could ensure the safety of the passengers. To this aim, thanks to the valve assembly, the breathable gas is not allowed to feed the breathing mask when the breathing mask is in the stowage box but the breathable gas is automatically allowed to feed the breathing mask when the breathing mask is used.
The breathing assembly disclosed in WO-2009/007794 avoids feeding the breathing mask when stored. Indeed, previously the breathing assembly was deprived of a valve assembly and the mask regulator was constantly pressurised.